What are you working with?
Drain holes in the clutch housings of many of these old tractors have a cotter pin in them that jiggles around during tractor operation and thereby keeps the hole open.
In that situation, a engine oil leak at the engine's rear crankshaft seal or transmission oil leaking forward into the clutch housing will find it's way down to the drain hole and "mark" the tractor's territory.
Today, we expect everything to be clean and sanitary, but "back in the day" nobody freaked out about a few drips of oil on the ground/floor every now and then.
NOT sure what you are looking at the has a cotter key/pin in a drain PLUG, although I have seem various machinery with a fill/vent plug with a cotter key through between flats to keep the vent hole open, just like the previously mentioned keys in drain holes.